This invention relates generally to lithography and, more particularly, to masks for use in optical lithography at or below about 180 nm.
Lithography is a process for producing a pattern on a semiconductor wafer. The pattern is produced by first exposing a pattern etched into a mask onto a semiconductor wafer coated with a resist material. The projected image of the pattern changes the composition of the resist material on the semiconductor wafer which is then removed to leave a matching pattern on the semiconductor wafer for further processing.
Depending upon the particular lithography application, the mask needs to satisfy several different requirements. The challenge is in finding a material or materials for use as the mask which will satisfy these requirements.
Historically, optical masking has been performed using a layer of chromium rich chromium nitride over fused silica, overcoated with a chromium oxinitride anti-reflective (xe2x80x9cARxe2x80x9d) layer to reduce reflectivity below 10%. Chromium (xe2x80x9cCrxe2x80x9d) is an excellent choice from optical and thermomechanical standpoints at wavelengths where it is sufficiently opaque, i.e. wavelengths generally above 180 nm. The spectra of chromium metal shown in FIG. 1 illustrates why it has been such a good mask from about 193 nm to about 436 nm. Referring to FIG. 2, the optical density (plotted here at log (transmission)) of a 1000 xc3x85 Cr metal film is shown. Again for wavelengths between about 193 nm and 500 nm, chromium is an excellent choice. The optical density of Cr film is near 5.0 in the ultraviolet (xe2x80x9cUVxe2x80x9d) range and it decreases to xcx9c4.5 at 193 nm. As thinner Cr metal films are used in manufacturing (800 xc3x85 for instance), the optical density above 190 nm may still be sufficiently high (above 4.0).
However, as shown in FIG. 1, chromium metal is a less desirable choice as a mask for wavelengths below 193 nm. Additionally, as shown in FIG. 2, at 157 nm, the optical density of the Cr metal film is 3.5 for a 1000 xc3x85 film and as low as 3.0 for a 800 xc3x85 film. At these optical densities, mask modulation is likely too be low for imaging of fine feature geometry. Unfortunately, a masking film thickness of over about 800 xc3x85 is undesirable for 157 nm applications because of the aspect ratio requirements of features smaller than 300 nm. Accordingly, it appears that chromium mask materials may not be adequate below 193 nm, in particular at potential vacuum ultraviolet (xe2x80x9cVUVxe2x80x9d) wavelengths, i.e. from about 120 nm to 180 nm, such as 157 nm.
Another factor in selecting a mask for use in optical lithography below 180 nm is the etch characteristics of the mask. The mask must have suitable etch characteristics with selectivity to the underlying substrate and to the resist material. In other words, the film on the substrate which forms the mask must be made of a material or materials which can be etched to form the pattern to be replicated on semiconductor wafers without significant loss to the underlying substrate or to the resist material.
As yet, an appropriate replacement material or materials for chromium films for use as the mask for lithography at or below 180 nm, i.e. a mask with the desired optical properties and etch characteristics, has not been found.
A mask for use on a layer of imaging material which is located on at least a portion of one surface of a substrate in a lithography process in accordance with one embodiment of the present invention includes a layer of a masking material which has an optical density of at least 4.0 for wavelengths at or below about 180 nm and a thickness equal to or less than about 1000 angstroms. Materials, such as tungsten and amorphous silicon, can be used for the mask.
A lithography system in accordance with another embodiment of the present invention includes a substrate with at least one surface, a layer of imaging material on at least a portion of the one surface, and a layer of masking material which has an optical density of at least 4.0 for wavelengths at or below about 180 nm and a thickness equal to or less than about 1000 angstroms on at least a portion of the layer of imaging material.
A method for lithography in accordance with another embodiment of the present invention includes a few steps. First, a mask is applied over at least a portion of one surface of a substrate, wherein the mask has an optical density of at least 4.0 for wavelengths at or below about 180 nm and a thickness equal to or less than about 1000 angstroms. Next, a layer of imaging material is applied over at least a portion of the mask. Next, at least a portion of the layer of imaging material and the mask are etched and then the remaining portion of the layer of imaging material is removed. The mask is then exposed to radiation at wavelengths at or below about 180 nm.
The present invention provides a number of advantages including providing a mask which has desirable optical properties for use in optical lithography at or below about 180 nm. More specifically, the mask has an optical density of at least 4.0 for wavelengths at or below about 180 nm with a thickness equal to or less than about 1000 angstroms.
The present invention also provides a mask with suitable etch characteristics. The layer of masking material can be etched without a significant loss of the underlying substrate or resist material.